“Core” body temperature is the temperature of the vital organs of a person or animal. An abnormally elevated core body temperature occurs when an individual is in a febrile or hyperthermic state and can result in seizures, protein denaturation and even cell death. An abnormally low core body temperature causes an individual to be in a hypothermic state, which can affect and impair the rate at which chemical reactions in the body take place and possibly lead to respiratory or circulatory failure.
Accurate core body temperature measurements are clearly important for medical diagnosis in clinics, emergency rooms and operating rooms. Additionally, there are many occupations that can put the individual at risk for hypothermia or hyperthermia including: fire fighters, solders, athletes, military pilots and divers. There is a clear need for an accurate and non-invasive way to measure core body temperature.
The standard for “normal” core temperature of the human body is 37° C. or 98.7° F. However, research has shown that “normal” is actually a range of 37°+/−1° C. (96.8°-100.4° F.). Typically the body maintains its core temperature within a narrow range of 37°+/−0.5° C. In fact, vasomotor responses to changes in core body temperature can be detected with as little as a 0.1° C. deviation from baseline.
The temperature of the skin is generally recognized as being 3-10° C. cooler than the core body temperature. The actual gradient between skin and core temperatures depends on many factors including: the person's core temperature, ambient environmental temperature, wind and air movement, sweating or skin wetness for other reasons, the level of physical exertion, where on the body the skin is located, clothing, hydration and vasomotor tone.
The human body and all mammals have a “core” thermal compartment and a “peripheral” thermal compartment. The core compartment consists roughly of the central volume of the torso, head and neck. The core compartment includes all of the major organs. The operation of most physiologic functions is exquisitely sensitive to variations in temperature. Most physiologic functions are chemical reactions and chemical reactions are well known to be temperature dependent. Enzyme mediated or enzyme enhanced chemical reactions are even more temperature sensitive. Enzyme mediated chemical reactions slow dramatically with as little as a 1-2° C. drop in temperature. Therefore, protecting the core temperature and thus the temperature of the vital organs where chemical reactions sustain life, is paramount to survival.
The core compartment is surrounded by a thermally insulating layer of skin, subcutaneous tissue and fat, known collectively as the peripheral thermal compartment. Further, the entire volume of the arms and legs are in the peripheral thermal compartment. The purpose of the peripheral compartment is to thermally insulate the core compartment from environmental thermal stress, especially cold. By controlling the blood flow to the peripheral compartment, the temperature of the peripheral compartment is “allowed” to vary widely and sacrifice its heat in order to protect the temperature of the core compartment. For example, on a cold day with inadequate clothing, the skin and subcutaneous tissue surrounding the torso may be severely vasoconstricted, reducing the local blood flow and causing the peripheral compartment to be 5° C. or more cooler that the temperature of the core compartment directly beneath it. The peripheral compartment becomes a thermal insulator between the core compartment and the environment. The skin of the lower legs may be more that 10° C. colder than the core temperature on a cold day.
On a warm day, especially with exercise, the skin of the peripheral compartment may be equal to the temperature of the core compartment. In a hot environment, the tissues of the peripheral compartment are vasodilated causing an increase in skin blood flow in order to promote heat dissipation through the skin.
The temperature of the peripheral compartment is easy to assess with skin surface temperature measurements. However, as previously discussed, the temperature of the peripheral compartment varies widely and unpredictably relative to core temperature, depending on the ambient environmental temperature as well as many other external and internal factors. Therefore, the temperature of the skin and peripheral compartment is an unreliable indicator of core temperature.
At the present time, all true core temperature measurements are invasive. For example core temperatures can be directly measured in the: pulmonary artery, esophagus, rectum, bladder and tympanic membrane. Measuring the temperature in these locations is necessarily invasive and may be risky and therefore these locations are not suitable for routine temperature measurements.
A wide variety of other less invasive temperature monitoring sites have been tried. These include: oral, nasal, infrared (IR) tympanic membrane emissions, axillary and forehead skin. All of these sites have been shown to be influenced in an unpredictable way by peripheral compartment and ambient environmental temperatures. Therefore, these non-invasive measurements have proven to be inaccurate or unreliable indicators of core temperature.
A variety of thermometers have been developed that measure the skin temperature and the ambient environmental temperature and then use a mathematical calculation which is supposed to compensate for the affect of ambient environmental temperature on the peripheral compartment. For example, an existing device uses two thermistors that measure skin and ambient environmental temperature respectively. A microprocessor calculates a compensation factor for the effect of the ambient temperature on the skin. Another device comprises an infrared thermometer that estimates core body temperature by measuring the axillary or tympanic membrane temperature by IR emission. The device then calculates core body temperature using the arterial heat balance equation which is based on heat flow through a thermal resistance from the core to a location of temperature measurement such as the skin and then to the ambient environmental temperature. The core temperature is based on skin temperature with a compensation factor for the ambient environmental temperature. Neither of these techniques have proven to be reliable or accurate.
A proposed device comprises a thermometer with two temperature sensors and thermal insulation between the two sensors forming a heat flux transducer. A heat flux transducer measures heat flow. This device uses an additional layer of thermal insulation between the skin and the environment in order to allow the skin to eventually equilibrate to zero heat flux with the environment and supposedly is in simultaneous equilibrium with the core body temperature. It is doubtful that such a device can accurately and reliably measure core temperature but if it could, it would require a long time to reach thermal equilibrium between the various thermal compartments and the ambient environment.
Several devices use an overlaying heater that is carefully controlled to equal skin temperature, thus allowing the heat from the core to migrate to the skin surface but not be lost to the ambient environment. The overlaying heater adjusted to skin temperature is essentially a perfect insulator. One of these devices comprises a deep tissue thermometer that has two temperature sensors separated by a known thermal insulation layer forming a heat flux transducer. A heater overlays the temperature sensors and is controlled by the servo-controller to “null heat flux” meaning that the temperature of the heater is precisely maintained at skin temperature so that there is zero heat flow from the skin to the ambient environment. Since heat loss is prevented, the skin eventually equilibrates with the deeper core tissue as the core heat slowly migrates to the skin surface. Equilibration takes 15 minutes or more.
Other devices also comprise deep tissue thermometers with two sensors and an overlaying heater that is controlled to precisely equal skin temperature, thus preventing any heat loss to the ambient environment. Unlike devices which use a heat flux transducer to detect heat flow, these devices measure heater temperature directly and simply match it with the skin temperature resulting from heat migrating outward from the core thermal compartment.
All of these thermometers that include overlaying heaters operate the heater throughout the temperature measurement. Additionally, each of these devices carefully controls the temperature of the heater to match the skin temperature and thus the heater serves as a perfect thermal insulator between the skin and the ambient environment. However, since the heater is carefully controlled to equal skin temperature, the heater by definition, does not actively heat the skin. All of these thermometers require two temperature sensors, one for the skin and one for the heater. Finally, all of these thermometers require equilibration of the core temperature with the peripheral compartment and then eventually equilibration with the skin, which can take a relatively long time. The long equilibration time of 15 minutes or more makes these thermometers impractical for most core temperature measurement indications.
In summary, core body temperature can be measured directly but the measurement techniques are invasive, cumbersome or risky. Peripheral thermal compartment temperatures can be measured non-invasively and directly but are not accurate or reliable indicators of core body temperature. Peripheral temperature measured with heat flux transducers and overlaying heaters that are carefully controlled to equal the skin temperature, allow eventual thermal equilibrium between the core thermal compartment, peripheral thermal compartment and skin surface. These thermometers have been shown to have a good correlation with core body temperature but are very slow, expensive and complicated.
A reliable, non-invasive, accurate, inexpensive and fast (for example; less than 3 minutes) device for measuring core body temperature from the skin surface is needed.